1. Field of the Invention
The present invention relates to a communication apparatus, communication method, and communication system.
2. Description of the Related Art
There is a computing system which arranges a display terminal having a minimum of input/output interfaces on the user side and causes a main body apparatus located in a remote site to execute complex arithmetic processes for a purpose of improving usability.
For example, a system which projects the screen information of a main body apparatus (e.g., personal computer or server computer) on a display terminal via a network (e.g., U.S. Pat. No. 6,784,855) is known. In the system disclosed in U.S. Pat. No. 6,784,855, input information (e.g., pen input by a digitizer) at the display terminal is transmitted to the main body apparatus through the network. The main body apparatus executes an application program process for the input information and transmits the execution result of the process and screen update information to the display terminal through the network. The display terminal executes a rendering process on the basis of the received screen update information.
VNC (Virtual Network Computing) is known as a technique of efficiently transmitting the screen information of a main body apparatus on a remote network from the main body apparatus to a terminal apparatus. In VNC, upon detecting that the screen is updated, the value of readout pixel information is compared with that of pixel information precedingly transmitted to the display terminal, thereby determining the updated screen area which has changed from the preceding state. The updated screen area undergoes still image compression, and only the compressed difference screen information is transmitted to the display terminal. This suppresses consumption of the communication band. In VNC, if the screen largely changes (e.g., when a window moves), the screen information amount to be transmitted increases. To the contrary, if the screen change is small, the screen information amount to be transmitted decreases.
When the above system is implemented using, e.g., a wireless LAN complying with the IEEE802.11 standard, power is wasted even while the display terminal is not executing transmission and reception processes.
However, U.S. Pat. No. 6,784,855 does not deal with a technique associated with power saving.
Regarding the power saving method of IEEE802.11, a terminal has two operation modes: an active mode which is always in communication enable state and a power save mode which periodically switches between communication enable and disable states. In the power save mode, the terminal supplies power to the transmission/reception system of the wireless communication processing unit to shift to an awake state every interval of receiving a beacon frame from an access point. Except that period, the terminal shifts to a doze state and operates using minimum necessary power. A data frame directed to the terminal operating in the power save mode is buffered at an access point. The access point cannot transmit the data frame until a transmission request frame is received from the terminal. Hence, if the power saving control of IEEE802.11 is simply applied, data are accumulated in the buffer of the access point, and buffer overflow (i.e., frame loss) occurs.
JP-A 2002-323942(KOKAI) discloses a system including a computer and wireless display which can wirelessly communicate with each other. In this system, occurrence of a specific event (e.g., mail arrival or planned time of a schedule) in the computer is detected, and the operation state of the wireless display is switched from the power save state (power save mode) to the normal state (active mode).
However, JP-A 2002-323942(KOKAI) does not consider operation state notification control between the terminal and an access point assuming a wireless LAN of IEEE802.11 in association with power saving of the wireless communication processing unit. Hence, for example, even after the terminal shifts from the power save state to the normal state, the access point recognizes that the terminal is still in the power save state and continues to buffer frames directed to the terminal, resulting in buffer overflow (i.e., frame loss).
As described above, conventionally, when a communication apparatus transmits data to a display terminal normally operating in the power save state through an access point, the data from the communication apparatus to the display terminal may be lost at the access point.